Local area networks, LANs, have been designed with many different addressing schemes. For example, the Ethernet LAN design, IEEE 802.3 standard, has 47 bits designated for multicast addresses for a total of approximately 140 trillion addresses.
In contrast, implementations of the token ring IEEE 802.5 standard provide only approximately 24 to approximately 31 separate addresses for multicast addresses, and these addresses are provided in the functional addressing defined by the IEEE 802.5 standard. The IEEE 802.5 standard provides 6 bytes, or 48 bits, for functional addressing, but chips implementing the standard first require a few of the bits to be set to a defined value, referred to as the Functional Address Indicator, and also require that only one bit of the remaining bits be set to "1", and the others be set to "0". The bits having the defined value give an indication that the rest of the bits indicate a "functional address", and the "rest of the bits" is only approximately 24 to 31 bits depending upon the implementation, and only one of these bits can be set.
It is common practice to forward frames from one LAN to another LAN by means of a bridge. Normally, the destination address field of a frame arriving at the bridge from a first LAN is copied into the destination address field of the frame transmitted by the bridge onto the second LAN. However, in the case of multicast addresses, in the event that the first LAN is an Ethernet supporting 140 trillion multicast addresses it is not possible to directly map to a token ring IEEE 802.5 address space of only approximately 24 to 31 addresses available for multicast purposes.
A solution to the mapping of multicast addresses between LANs of different size multicast address spaces has been suggested in U.S. Pat. No. 4,933,938, issued to D. B. Sheehy on Jun. 12, 1990. In the Sheehy patent a translation table is used to directly map the multicast addresses of the first LAN onto the multicast addresses of the second LAN.
However, there still remains the problem of some LANs such as Ethernet having approximately 140 trillion multicast addresses and other LANs such as the token ring IEEE 802.5 standard LAN having only approximately 24 to 31 multicast addresses in their functional address space. In bridging an Ethernet LAN to a token ring 802.5 LAN, much of the advantage of having many multicast addresses in Ethernet will be lost in the forwarding of frames to the token ring IEEE 802.5 standard LAN.